We wish to understand on a molecular level enzyme catalysis in which molecular oxygen is cleaved and incorporated into organic substrates. We have chosen a representative group of iron containing di- and mono-oxygenases which include protocatechuate 3,4 dioxygenase and cytochrome P450. We are using the systematic application of diverse techniques to describe mechanisms at every level of protein structure; these include: optical, MCD, ENDOR, EPR, NMR and Mossbauer spectroscopy; chemical modification, amino acid sequence determination; low temperature, steadystate and presteady state kinetics. Our goals are 1) to formulate reasonable and defensible mechanisms at the molecular level, 2) to identify the form of oxygen which reacts with substrates, 3) to understand the role of iron in these metalloproteins, 4) to understand the bearing on catalysis of protein interactions with other proteins, solvent and membranes.